Substrate-induced modulation of glutamate uptake in human platelets

The intriguing role of platelets as custodians of brain-derived neurotrophic factor

A State of the Art lecture titled "Platelets and neurotrophins" was presented at the International Society on Thrombosis and Haemostasis Congress in 2023. Neurotrophins, a family of neuronal growth factors known to support cognitive function, are increasingly recognized as important players in vascular health. Indeed, along with their canonical receptors, neurotrophins are expressed in peripheral tissues, particularly in the vasculature. The better-characterized neurotrophin in vascular biology is the brain-derived neurotrophic factor (BDNF). Its largest extracerebral pool resides within platelets, partly inherited from megakaryocytes and also likely internalized from circulation. Activation of platelets releases vast amounts of BDNF into their milieu and interestingly leads to platelet aggregation through binding of its receptor, the tropomyosin-related kinase B, on the platelet surface. As BDNF is readily available in plasma, a mechanism to preclude excessive platelet activation and aggregation appears critical. As such, binding of BDNF to α2-macroglobulin hinders its ability to bind its receptor and limits its platelet-activating effects to the site of vascular injury. Altogether, addition of BDNF to a forming clot facilitates not only paracrine platelet activation but also binding to fibrinogen, rendering the resulting clot more porous and plasma-permeable. Importantly, release of BDNF into circulation also appears to be protective against adverse cardiovascular and cerebrovascular outcomes, which has been reported in both animal models and epidemiologic studies. This opens an avenue for platelet-based strategies to deliver BDNF to vascular lesions and facilitate wound healing through its regenerative properties. Finally, we summarize relevant new data on this topic presented during the 2023 International Society on Thrombosis and Haemostasis Congress.

Glutamate Receptor Dysregulation and Platelet Glutamate Dynamics in Alzheimer's and Parkinson's Diseases: Insights into Current Medications

Two of the most prevalent neurodegenerative disorders (NDDs), Alzheimer's disease (AD) and Parkinson's disease (PD), present significant challenges to healthcare systems worldwide. While the etiologies of AD and PD differ, both diseases share commonalities in synaptic dysfunction, thereby focusing attention on the role of neurotransmitters. The possible functions that platelets may play in neurodegenerative illnesses including PD and AD are becoming more acknowledged. In AD, platelets have been investigated for their ability to generate amyloid-ß (Aß) peptides, contributing to the formation of neurotoxic plaques. Moreover, platelets are considered biomarkers for early AD diagnosis. In PD, platelets have been studied for their involvement in oxidative stress and mitochondrial dysfunction, which are key factors in the disease's pathogenesis. Emerging research shows that platelets, which release glutamate upon activation, also play a role in these disorders. Decreased glutamate uptake in platelets has been observed in Alzheimer's and Parkinson's patients, pointing to a systemic dysfunction in glutamate handling. This paper aims to elucidate the critical role that glutamate receptors play in the pathophysiology of both AD and PD. Utilizing data from clinical trials, animal models, and cellular studies, we reviewed how glutamate receptors dysfunction contributes to neurodegenerative (ND) processes such as excitotoxicity, synaptic loss, and cognitive impairment. The paper also reviews all current medications including glutamate receptor antagonists for AD and PD, highlighting their mode of action and limitations. A deeper understanding of glutamate receptor involvement including its systemic regulation by platelets could open new avenues for more effective treatments, potentially slowing disease progression and improving patient outcomes.

The multifaceted role of platelets in mediating brain function

Platelets, the small, anucleate blood cells that originate from megakaryocytes in the bone marrow, are typically associated with coagulation. However, it is now apparent that platelets are more multifaceted than originally thought, with their function extending beyond their traditional role in hemostasis to acting as important mediators of brain function. In this review, we outline the broad repertoire of platelet function in the central nervous system, focusing on the similarities between platelets and neurons. We also summarize the role that platelets play in the pathophysiology of various neurological diseases, with a particular focus on neuroinflammation and neurodegeneration. Finally, we highlight the exciting prospect of harnessing the unique features of the platelet proteome and extracellular vesicles, which are rich in neurotrophic, antioxidative, and antiinflammatory factors, for the development of novel neuroprotective and neuroregenerative interventions to treat various neurodegenerative and traumatic pathologies.

Effects of Nutrients on Platelet Function: A Modifiable Link between Metabolic Syndrome and Neurodegeneration?

Metabolic syndrome increases the risk of vascular dementia and other neurodegenerative disorders. Recent studies underline that platelets play an important role in linking peripheral with central metabolic and inflammatory mechanisms. In this narrative review, we address the activation of platelets in metabolic syndrome, their effects on neuronal processes and the role of the mediators (e.g., serotonin, platelet-derived growth factor). Emerging evidence shows that nutritional compounds and their metabolites modulate these interactions-specifically, long chain fatty acids, endocannabinoids and phenolic compounds. We reviewed the role of activated platelets in neurovascular processes and nutritional compounds in platelet activation.

Study on antidepressant mechanism of Radix Bupleuri-Radix Paeoniae Alba herb pair by metabonomics combined with 1H nuclear magnetic resonance and ultra-high-performance liquid chromatography-tandem mass spectrometry detection technology

OBJECTIVES

Radix Bupleuri-Radix Paeoniae Alba (BP), a traditional Chinese medicine herb pair, has treated depression by coordinating the liver in Chinese classical medicine books and modern research. This study aims to verify the antidepressant effect of BP by behavioural examination, and reveal the underlying antidepressant mechanisms of BP.

METHODS

The antidepressant effects in chronic unpredictable mild stress (CUMS) of BP were observed by behavioural indicators and 1H nuclear magnetic resonance (1H-NMR) and ultra-high-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) metabonomics techniques combined with the related analysis platforms.

KEY FINDINGS

BP could significantly improve the depressive behaviour of CUMS rats. Compared with the model group, body weight (P < 0.05), the number of crossing (P < 0.001) and rearing (P < 0.01) and sucrose preference rate (P < 0.01) were significantly enhanced, and the immobility time was shortened in the forced swimming test (P < 0.001) of the BP group. In metabonomics study, 35 depression-related metabolites were identified by 1H NMR and UHPLC-MS/MS metabonomics by comparing model and control groups. BP could significantly retrieve 17 depression-related metabolites. Thirteen depression-related metabolic pathways were found through Met-PA and BP could regulate seven metabolic pathways.

CONCLUSIONS

BP herb pair had significantly antidepressant effect, which provides a basis for further finding drug targets.

Ionotropic glutamate receptors in platelets: opposing effects and a unifying hypothesis

Ionotropic glutamate receptors include α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPAR), kainate receptors (KAR), and N-methyl-D-aspartate receptors (NMDAR). All function as cation channels; AMPAR and KAR are more permeable to sodium and NMDAR to calcium ions. Compared to the brain, receptor assemblies in platelets are unusual, suggesting distinctive functionalities.There is convincing evidence that AMPAR and KAR amplify platelet function and thrombus formation in vitro and in vivo. Transgenic mice lacking GluA1 and GluK2 (AMPAR and KAR subunits, respectively) have longer bleeding times and prolonged time to thrombosis in an arterial model. In humans, rs465566 KAR gene polymorphism associates with altered in vitro platelet responses suggesting enhanced aspirin effect. The NMDAR contribution to platelet function is less well defined. NMDA at low concentrations (≤10 μM) inhibits platelet aggregation and high concentrations (≥100 μM) have no effect. However, open NMDAR channel blockers interfere with platelet activation and aggregation induced by other agonists in vitro; anti-GluN1 antibodies interfere with thrombus formation under high shear rates ex vivo; and rats vaccinated with GluN1 develop iron deficiency anemia suggestive of mild chronic bleeding. In this review, we summarize data on glutamate receptors in platelets and propose a unifying model that reconciles some of the opposing effects observed.

Accelerated capacity of glutamate uptake via blood elements as a possible tool of rapid remote conditioning mediated tissue protection

Recently, the function of blood cells in remote ischemic conditioning (RIC) mediated neuroprotection was undoubtedly confirmed. In the present paper, we have focused on the role of blood elements in glutamate homeostasis. The blood of remote conditioned (tolerant) animals was incubated ex vivo with 100 μM glutamate, and the quantitative and qualitative changes of excitatory amino acid transporters (EAAT 1, 2, and 3) were determined. We confirmed RIC mediated accelerated sequestration of extracellular glutamate via EAATs and altered distribution of that amino acid between plasma and cell elements compared to non-tolerant counterparts. The activity of EAATs was elevated in erythrocytes and monocytes, while the density of transporters was not affected. Quantitative changes of EAAT1 density were detected solely in platelets where the forced scavenging was independent of EAATs inhibition. Surprisingly, the trafficking of immunovisualised EAAT2 and 3 raised at tolerant erythrocytes and monocytes. We have found that protein synthesis underlined this process. On the other hand, depletion of protein synthesis did not significantly affect the scavenging capacity of those cell populations. Our work has demonstrated that the elevated blood scavenging of glutamate overdose could be one of the potential mechanisms underlying RIC mediated tissue protection.

Platelets in Neurodegenerative Conditions-Friend or Foe?

It is now apparent that platelet function is more diverse than originally thought, shifting the view of platelets from blood cells involved in hemostasis and wound healing to major contributors to numerous regulatory processes across different tissues. Given their intriguing ability to store, produce and release distinct subsets of bioactive molecules, including intercellular signaling molecules and neurotransmitters, platelets may play an important role in orchestrating healthy brain function. Conversely, a number of neurodegenerative conditions have recently been associated with platelet dysfunction, further highlighting the tissue-independent role of these cells. In this review we summarize the requirements for platelet-neural cell communication with a focus on neurodegenerative diseases, and discuss the therapeutic potential of healthy platelets and the proteins which they release to counteract these conditions.

Platelet glutamate uptake and Th1 cells inversely correlate in relapsing/remitting and in progressive multiple sclerosis

BACKGROUND

High affinity sodium-dependent Excitatory Amino Acid Transporters (EAAT), present in glial and neuron cells, clear around 90% of the synaptic cleft released glutamate, and their impaired activity seem to be critical for many neurodegenerative disorders, including Multiple Sclerosis (MS). These transporters are also present in human platelets, and they show molecular and biochemical characteristics similar to those in the CNS.

OBJECTIVES

The aim of this study was to investigate whether EAAT-dependent uptake is present also at the peripheral level in blood of MS patients. Moreover, since platelets (plt) and peripheral blood mononuclear cells (PBMC) share the same intra-corporeal fluid, they might be reciprocally influenced, and the glutamate uptake modulation might be useful as a peripheral "trait-marker" to characterize different clinical courses of MS RESULTS: : Reduced uptake values were found in MS patients compared to healthy controls (HC), as well as significant differences were found across MS clinical courses. Representative saturation curves showed that Vmax was significantly decreased for patients compared to HC. Conversely, dissociation constant of the two reactions appeared similar for MS and HC subjects. Furthermore, clinical forms of MS with mild (benign) prognosis was not affected as fa as concern EAAT uptake. Gender, age, and drug treatments did not impact glutamate uptake efficiency. Interestingly, a negative correlation between EAAT activity and percentage of Th1 cells (CD4+IFNγ+ and CD4+TBET+IFNγ+ cells) was observed, suggesting a relationship between EAAT impairment and a pro-inflammatory environment.

CONCLUSIONS

Interestingly, as shown in the CNS, a relationship between clinical, inflammatory MS features and glutamate clearance can be also assessed in platelets. Moreover, glutamate uptake activity might be an useful biomarker to characterize patients with benign prognosis.

Can a Selective Serotonin Reuptake Inhibitor Act as a Glutamatergic Modulator?

Sertraline (Zoloft) and fluoxetine (Prozac) are selective serotonin reuptake inhibitors whose antidepressant mechanism of action is classically attributed to an elevation of the extracellular levels of serotonin in the synaptic cleft. However, the biological effects of these drugs seem to be more complex than their traditionally described mechanism of action. Among their actions is the inhibition of different types of Na⁺ and K⁺ channels, as well as of glutamate uptake activity. The clearance of extracellular glutamate is essential to maintain the central nervous system within physiological conditions, and this excitatory neurotransmitter is removed from the synaptic cleft by astrocyte transporters. This transport depends upon a hyperpolarized membrane potential in astrocytes that is mainly maintained by Kir4.1 K⁺ channels. The impairment of the Kir4.1 channel activity reduces driving force for the glutamate transporter, resulting in an accumulation of extracellular glutamate. It has been shown that sertraline and fluoxetine inhibit Kir4.1 K⁺ channels. Recently, we demonstrated that sertraline reduces glutamate uptake in human platelets, which contain a high-affinity Na⁺-dependent glutamate uptake system, with kinetic and pharmacological properties similar to astrocytes in the central nervous system. Considering these similarities between human platelets and astrocytes, one might ask if sertraline could potentially reduce glutamate clearance in the synaptic cleft and consequently modulate glutamatergic transmission. This possibility merits investigation, since it may provide additional information regarding the mechanism of action and perhaps the side effects of these antidepressants.

Inhibition of NMDA receptor function with an anti-GluN1-S2 antibody impairs human platelet function and thrombosis

GluN1 is a mandatory component of N-methyl-D-aspartate receptors (NMDARs) best known for their roles in the brain, but with increasing evidence for relevance in peripheral tissues, including platelets. Certain anti-GluN1 antibodies reduce brain infarcts in rodent models of ischaemic stroke. There is also evidence that human anti-GluN1 autoantibodies reduce neuronal damage in stroke patients, but the underlying mechanism is unclear. This study investigated whether anti-GluN1-mediated neuroprotection involves inhibition of platelet function. Four commercial anti-GluN1 antibodies were screened for their abilities to inhibit human platelet aggregation. Haematological parameters were examined in rats vaccinated with GluN1. Platelet effects of a mouse monoclonal antibody targeting the glycine-binding region of GluN1 (GluN1-S2) were tested in assays of platelet activation, aggregation and thrombus formation. The epitope of anti-GluN1-S2 was mapped and the mechanism of antibody action modelled using crystal structures of GluN1. Our work found that rats vaccinated with GluN1 had a mildly prolonged bleeding time and carried antibodies targeting mostly GluN1-S2. The monoclonal anti-GluN1-S2 antibody (from BD Biosciences) inhibited activation and aggregation of human platelets in the presence of adrenaline, adenosine diphosphate, collagen, thrombin and a protease-activated receptor 1-activating peptide. When human blood was flowed over collagen-coated surfaces, anti-GluN1-S2 impaired thrombus growth and stability. The epitope of anti-GluN1-S2 was mapped to α-helix H located within the glycine-binding clamshell of GluN1, where the antibody binding was computationally predicted to impair opening of the NMDAR channel. Our results indicate that anti-GluN1-S2 inhibits function of human platelets, including dense granule release and thrombus growth. Findings add to the evidence that platelet NMDARs regulate thrombus formation and suggest a novel mechanism by which anti-GluN1 autoantibodies limit stroke-induced neuronal damage.

4-Аminopyridine sequesters intracellular Ca2+ which triggers exocytosis in excitable and non-excitable cells

4-aminopyridine is commonly used to stimulate neurotransmitter release resulting from sustained plasma membrane depolarization and Ca²⁺-influx from the extracellular space. This paper elucidated unconventional mechanism of 4-aminopyridine-stimulated glutamate release from neurons and non-neuronal cells which proceeds in the absence of external Ca²⁺. In brain nerve terminals, primary neurons and platelets 4-aminopyridine induced the exocytotic release of glutamate that was independent of external Ca²⁺ and was triggered by the sequestration of Ca²⁺ from intracellular stores. The initial level of 4-aminopyridine-stimulated glutamate release from neurons in the absence or presence of external Ca²⁺ was subequal and the difference was predominantly associated with subsequent tonic release of glutamate in Ca²⁺-supplemented medium. The increase in [Ca²⁺]_i and the secretion of glutamate stimulated by 4-aminopyridine in Ca²⁺-free conditions have resulted from Ca²⁺ efflux from endoplasmic reticulum and were abolished by intracellular free Ca²⁺ chelator BAPTA. This suggests that Ca²⁺ sequestration plays a profound role in the 4-aminopyridine-mediated stimulation of excitable and non-excitable cells. 4-Aminopyridine combines the properties of depolarizing agent with the ability to sequester intracellular Ca²⁺. The study unmasks additional mechanism of action of 4-aminopyridine, an active substance of drugs for treatment of multiple sclerosis and conditions related to reduced Ca²⁺ efflux from intracellular stores.

Platelet Glutamate Uptake and Release in Migraine With and Without Aura

Glutamate may play an important role in the pathogenesis of migraine: glutamate release in the brain may be involved in the development of spreading depression and increased concentrations of this amino acid have been reported in plasma and platelets from migraine patients. Here we assessed platelet glutamate uptake and release in 25 patients affected by migraine with aura (MA) and 25 patients affected by migraine without aura (MoA), comparing the results with a group of 20 healthy matched controls. Both glutamate release from stimulated platelets and plasma concentrations of the amino acid were assessed by high-performance liquid chromatography, and were increased in both types of migraine, although more markedly in MA. Platelet glutamate uptake, assessed as 3H-glutamate intake, was increased in MA, while it was reduced in MoA with respect to the control group. These results support the view that MA might involve different pathophysiological mechanisms from MoA and, specifically, up-regulation of the glutamatergic metabolism. Understanding these dysfunctional pathways could lead to new, possibly more successful therapeutic approaches to the management of migraine.

Sertraline reduces glutamate uptake in human platelets

Mitochondrial damage and declines in ATP levels have been recently attributed to sertraline. The effects of sertraline on different parameters were investigated in washed platelets from 18 healthy male volunteers, after 24h of drug exposure. Sertraline toxicity was observed only at the highest concentrations, 30 and 100 μM, which significantly reduced platelet viability to 76 ± 3% and 20 ± 2%, respectively. The same concentrations significantly decreased total ATP to 73 ± 3% and 13 ± 2%, respectively. Basal values of glycogen were not significantly affected by sertraline treatment. Glutamate uptake was significantly reduced after treatment with 3, 30 and 100 μM, by 28 ± 6%, 32 ± 5% and 54 ± 4%, respectively. Our data showed that sertraline at therapeutic concentrations does not compromise platelet viability and ATP levels, but they suggest that in a situation where extracellular glutamate levels are potentially increased, sertraline might aggravate an excitotoxic condition.

N-methyl-D-aspartate receptors amplify activation and aggregation of human platelets

BACKGROUND

Glutamate is stored in platelet dense granules and large amounts (>400 μM) are released during thrombus formation. N-methyl-d-aspartate glutamate receptors (NMDARs) have been shown in platelets but their roles are unclear.

MATERIALS AND METHODS

Platelet activation indices (CD62P expression and PAC-1 binding) and platelet aggregation were tested in the presence of well-characterized agonists (glutamate, NMDA, glycine) and antagonists (MK-801, memantine, AP5) of neuronal NMDARs. Expression of NMDAR subunits in platelets was determined.

RESULTS

NMDAR agonists facilitated and NMDAR antagonists inhibited platelet activation and aggregation. Low concentrations (100 μM) of MK-801 and memantine reduced adrenaline-induced CD62P expression by 47 ± 5 and 42 ± 3%, respectively, and inhibited adrenaline-induced platelet aggregation by 17 ± 6 and 25 ± 5%, respectively (P<0.05). AP5 caused less inhibition of platelet function, requiring concentrations of at least 250 μM to inhibit aggregation. NMDAR agonists did not aggregate platelets by themselves but enhanced aggregation initiated by low concentrations of ADP. Exogenous glutamate helped reverse inhibition of platelet aggregation by riluzole (inhibitor of glutamate release). Compared with seven possible NMDAR subunits in neurons, human platelets contained four: GluN1, GluN2A, GluN2D and GluN3A, a combination rarely seen in neurons. The presence of NMDAR transcripts in platelets implied platelet ability to regulate NMDAR expression presumably 'on demand'. Flow cytometry and electron microscopy demonstrated that in non-activated platelets, NMDAR subunits were contained inside platelets but relocated onto platelet blebs, filopodia and microparticles after platelet activation.

CONCLUSIONS

Our results support an active role for NMDARs in platelets, in a process that involves activation-dependent receptor relocation towards the platelet surface.

Role of platelets in neurodegenerative diseases: a universal pathophysiology

Platelets play an important role in a variety of disorders, namely, cardiovascular, psychosomatic, psychiatric, thrombosis, HIV/AIDS in addition to various neurodegenerative diseases (NDDs). Recent evidence indicates that platelet react to diverse stressors, thereby offering an interesting vantage point for understanding their potential role in contemporary medical research. This review addresses the possible role of platelets as a systemic probe in various NDDs, such as amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease, Alzheimer's disease, multiple sclerosis, etc. The current review based on published literature, describes a probable link between platelets and pathophysiology of various NDDs. It also discusses how platelets epitomize ultrastructural, morphological, biochemical and molecular changes, highlighting their emerging role as systemic tools in different NDDs.

Reduced urinary glutamate levels are associated with the frequency of migraine attacks in females

BACKGROUND AND PURPOSE

Recent evidences indicate that glutamatergic homeostasis disorders are implicated in the pathogenesis of migraine. In particular, plasma and cerebrospinal fluid glutamate levels seem to be altered in migraine patients. However, the impacts of glutamate on migraine and especially on aura symptoms, alterations in the frequency of migraine attacks as well as investigations on glutamate on migraine-related metabolic dysfunctions, like hyperinsulinaemia, and an atherogenic lipid profile remain elusive to date. The aim of the present study was to investigate the impact of glutamate on migraine and related metabolic dysfunctions.

METHODS

We investigated the urinary glutamate levels of female migraineurs (n = 48) in the interictal phase and healthy controls (n = 48). Parameters of the insulin- and lipid metabolism, inflammatory parameters and anthropometric parameters were additionally determined.

RESULTS

Urinary glutamate levels of female migraineurs were significantly decreased with respect to the control group. Logistic regression revealed an odds ratio of 4.04 for migraine. We found a significant correlation with the time-period of patients' last attack and a significant inverse correlation with the annual frequency of migraine attacks. Other parameters of the insulin- and lipid metabolism, anthropometric and inflammatory parameters showed no significant correlation with glutamate levels.

CONCLUSION

We show here that female migraineurs exhibit decreased urinary glutamate levels which are associated with a 4.04-fold higher risk for migraine and correlated with patients' frequency of migraine attacks.

High-fat diets induce changes in hippocampal glutamate metabolism and neurotransmission

Obesity and high-fat (HF) diets have a deleterious impact on hippocampal function and lead to impaired synaptic plasticity and learning deficits. Because all of these processes need an adequate glutamatergic transmission, we have hypothesized that nutritional imbalance triggered by these diets might eventually concern glutamate (Glu) neural pathways within the hippocampus. Glu is withdrawn from excitatory synapses by specific uptake mechanisms involving neuronal (EAAT-3) and glial (GLT-1, GLAST) transporters, which regulate the time that synaptically released Glu remains in the extracellular space and, consequently, the duration and location of postsynaptic receptor activation. The goal of the present study was to evaluate in mouse hippocampus the effect of a short-term high-fat dietary treatment on 1) Glu uptake kinetics, 2) the density of Glu carriers and Glu-degrading enzymes, 3) the density of Glu receptor subunits, and 4) synaptic transmission and plasticity. Here, we show that HF diet triggers a 50% decrease of the Michaelis-Menten constant together with a 300% increase of the maximal velocity of the uptake process. Glial Glu carriers GLT-1 and GLAST were upregulated in HF mice (32 and 27%, respectively), whereas Glu-degrading enzymes glutamine synthase and GABA-decarboxilase appeared to be downregulated in these animals. In addition, HF diet hippocampus displayed diminished basal synaptic transmission and hindered NMDA-induced long-term depression (NMDA-LTD). This was coincident with a reduced density of the NR2B subunit of NMDA receptors. All of these results are compatible with the development of leptin resistance within the hippocampus. Our data show that HF diets upregulate mechanisms involved in Glu clearance and simultaneously impair Glu metabolism. Neurochemical changes occur concomitantly with impaired basal synaptic transmission and reduced NMDA-LTD. Taken together, our results suggest that HF diets trigger neurochemical changes, leading to a desensitization of NMDA receptors within the hippocampus, which might account for cognitive deficits.

Towards a glutamate hypothesis of depression: an emerging frontier of neuropsychopharmacology for mood disorders. Neuropharmacology. 2012;62:63-77. [PMID: 21827775 DOI: 10.1016/j.neuropharm.2011.07.036]

Half a century after the first formulation of the monoamine hypothesis, compelling evidence implies that long-term changes in an array of brain areas and circuits mediating complex cognitive-emotional behaviors represent the biological underpinnings of mood/anxiety disorders. A large number of clinical studies suggest that pathophysiology is associated with dysfunction of the predominant glutamatergic system, malfunction in the mechanisms regulating clearance and metabolism of glutamate, and cytoarchitectural/morphological maladaptive changes in a number of brain areas mediating cognitive-emotional behaviors. Concurrently, a wealth of data from animal models have shown that different types of environmental stress enhance glutamate release/transmission in limbic/cortical areas and exert powerful structural effects, inducing dendritic remodeling, reduction of synapses and possibly volumetric reductions resembling those observed in depressed patients. Because a vast majority of neurons and synapses in these areas and circuits use glutamate as neurotransmitter, it would be limiting to maintain that glutamate is in some way 'involved' in mood/anxiety disorders; rather it should be recognized that the glutamatergic system is a primary mediator of psychiatric pathology and, potentially, also a final common pathway for the therapeutic action of antidepressant agents. A paradigm shift from a monoamine hypothesis of depression to a neuroplasticity hypothesis focused on glutamate may represent a substantial advancement in the working hypothesis that drives research for new drugs and therapies. Importantly, despite the availability of multiple classes of drugs with monoamine-based mechanisms of action, there remains a large percentage of patients who fail to achieve a sustained remission of depressive symptoms. The unmet need for improved pharmacotherapies for treatment-resistant depression means there is a large space for the development of new compounds with novel mechanisms of action such as glutamate transmission and related pathways. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Investigating GABA and its function in platelets as compared to neurons

We have recently suggested that platelets could be used as a model for neuronal receptors. In this paper we have investigated gamma-aminobutyric acid (GABA) metabolism and GABA receptors in platelets and in cultured neurons to see whether platelets' GABA mimics neuronal GABA receptor activities. We used the ELISA technique for detecting the GABA concentration in platelet rich plasma and cultured neurons. The functional effects of GABA and its receptor ligands on platelets were determined using an aggregometer. We found that the GABA concentration is 30% lower in platelets than in neurons and in both preparations GABA was metabolized by GABA transaminase (GABA-T). GABA potentiated calcium dependent platelet aggregation with a higher value in washed platelets suspension (WPS) then in platelet rich plasma (PRP). This effect was inhibited by benzodiazepines, calcium channel blockers and the selective phosphoinositide 3-kinase antagonist Wortmannin. GABA neurotransmission is involved in most aspects of normal brain function and can be perturbed in many neuropathologic conditions. We concluded that platelets could be further developed to be used as a peripheral model to study neuronal GABAergic function and its abnormality in diseases such as epilepsy and schizophrenia. Furthermore our results indicated that PI3-kinase is involved in calcium dependent GABA induced platelet aggregation as this synergistic effect is inhibited by Wortmannin in dose dependent manner.

Glutamate mediates platelet activation through the AMPA receptor

Glutamate is an excitatory neurotransmitter that binds to the kainate receptor, the N-methyl-D-aspartate (NMDA) receptor, and the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor (AMPAR). Each receptor was first characterized and cloned in the central nervous system (CNS). Glutamate is also present in the periphery, and glutamate receptors have been identified in nonneuronal tissues, including bone, heart, kidney, pancreas, and platelets. Platelets play a central role in normal thrombosis and hemostasis, as well as contributing greatly to diseases such as stroke and myocardial infarction. Despite the presence of glutamate in platelet granules, the role of glutamate during hemostasis is unknown. We now show that activated platelets release glutamate, that platelets express AMPAR subunits, and that glutamate increases agonist-induced platelet activation. Furthermore, we demonstrate that glutamate binding to the AMPAR increases intracellular sodium concentration and depolarizes platelets, which are important steps in platelet activation. In contrast, platelets treated with the AMPAR antagonist CNQX or platelets derived from GluR1 knockout mice are resistant to AMPA effects. Importantly, mice lacking GluR1 have a prolonged time to thrombosis in vivo. Our data identify glutamate as a regulator of platelet activation, and suggest that the AMPA receptor is a novel antithrombotic target.

Transport behavior and efflux of Rg1 in rat pulmonary epithelial cells

The aim of this study was to investigate whether ginsenoside Rg1 could be transported into rat pulmonary epithelial cells and its transport behavior and efflux through the cells. A high-performance liquid chromatography coupled with 2487 UV-vis detector at 203 nm was applied. The mobile phase was 0.05% phosphate-acetonitrile (75:25, v/v). Cells were incubated with Rg1 (100 microg/mL) for a specific time, then lysed and sonicated in methanol to extract intracellular Rg1. Cells incubated with Rg1 and verapamil or KCN were processed by the same method. A 20 microL aliquot of sample was injected into the HPLC system to determine Rg1 concentration. The results showed that Rg1 could be transported into the epithelial cells with peak concentration of 1.28 microg/10(5) cells at 0.5 h. Metabolic inhibitor KCN and P-glycoprotein inhibitor verapamil could increase Rg1 concentration within the cells, indicating that efflux of Rg1 was energy-dependent and P-gp was likely to be involved. This is the first time that the transport behavior and efflux of Rg1 through rat pulmonary epithelial cells has been demonstrated. The phenomenon that Rg1 concentration in the cells decreased whereas that in the medium remained high indicated that a more effective means of drug administration should be found.

Human platelets express the synaptic markers VGLUT1 and 2 and release glutamate following aggregation

Vesicular glutamate transporters (VGLUTs) are involved in storing glutamate for secretion at the level of glutamatergic axon terminals, and for this reason they have been extensively used as markers to identify glutamate-releasing cells. Platelets have been considered as a suitable model for studying glutamatergic dysfunction because they perform glutamate uptake and express both external transporters, and NMDA-like receptors. Here, we show that platelets express the pre-synaptic markers VGLUT1 and VGLUT2 and release glutamate following aggregation, implying a possible contributory role in the pathophysiology of stroke, migraine, and other excitotoxic disorders.

EAAC1 is expressed in rat and human prostate epithelial cells; functions as a high-affinity L-aspartate transporter; and is regulated by prolactin and testosterone

Background

Prostate epithelial cells accumulate a high level of aspartate that is utilized as a substrate for their unique function of production and secretion of enormously high levels of citrate. In most mammalian cells aspartate is synthesized; and, therefore is a non-essential amino acid. In contrast, in citrate-producing prostate cells, aspartate is an essential amino acid that must be derived from circulation. The prostate intracellular/extracellular conditions present a 40:1 concentration gradient. Therefore, these cells must possess a plasma membrane-associated aspartate uptake transport process to achieve their functional activity. In earlier kinetic studies we identified the existence of a unique Na+-dependent high-affinity L-aspartate transport process in rat prostate secretory epithelial cells. The present report is concerned with the identification of this putative L-aspartate transporter in rat and human prostate cells.

Results

The studies show for the first time that EAAC1 is expressed in normal rat prostate epithelial cells, in normal and hyperplastic human prostate glands, and in human malignant prostate cell lines. EAAC1 expression and high-affinity L-aspartate transport are correspondingly down-regulated by EAAC1 siRNA knock down. Exposure of prostate cells to physiological levels of prolactin or testosterone results in an up-regulation of EAAC1 expression and a corresponding increase in the high-affinity transport of L-aspartate into the cells.

Conclusion

This study shows that EAAC1 functions as the high-affinity L-aspartate transporter that is responsible for the uptake and accumulation of aspartate in prostate cells. In other cells (predominantly excitable tissue cells), EAAC1 has been reported to function as a glutamate transporter rather than as an aspartate transporter. The regulation of EAAC1 expression and L-aspartate transport by testosterone and prolactin is consistent with their regulation of citrate production in prostate cells. The identification of EAAC1 as the high-affinity L-aspartate transporter now permits studies to elucidate the mechanism of hormonal regulation of EAAC1 gene expression, and to investigate the mechanism by which the cellular environment effects the functioning of EAAC1 as an aspartate transporter or as a glutamate transporter.